Annu. Rev. Astron. Astrophys. 1997. 35: 503-556 Copyright © 1997 by Annual Reviews. All rights reserved

2. THE SOLAR IRON ABUNDANCE

It is sobering, and somewhat embarrassing, that the solar iron abundance is in dispute at the level of 0.15 dex. This discrepancy comes in spite of the fact that more than 2000 solar iron lines, with reasonably accurate gf values, are available for abundance analysis; that the solar spectrum is measured with much higher S/N and dispersion than for any other star; that LTE corrections to Fe I abundance are small, at only +0.03 dex (Holweger et al 1991); and that both theoretical and empirical solar model atmospheres are available, with parameters known more precisely than for any other star.

Anders & Grevesse (1989) reviewed published meteoritic and solar photospheric abundances for all available elements and found (Fe) = 7.51 ± 0.01 for meteorites and (Fe) = 7.67 from the solar abundance analysis of (Blackwell et al 1984, 1986), which was a notable increase from the earlier photospheric value of 7.50 ± 0.08 favored by Ross & Aller (1976). Blackwell et al's work utilized the Oxford group gf values for Fe I lines, which are known to be of high accuracy.

Pauls et al (1990) found (Fe) = 7.66 from Fe II lines, but Holweger et al (1990), also using Fe II lines, found (Fe) = 7.48. Biémont et al (1991) measured the solar iron abundance of 7.54 ± 0.03 with a larger sample of Fe II lines. Holweger et al (1991) found 7.50 ± 0.07 based on gf values for Fe I lines measured by Bard et al (1991).

Two recent papers are characteristic of the conflicting solar iron abundance: those by Holweger et al (1995), Blackwell et al (1995). Blackwell et al (1995) employed Oxford gf values and the Holweger & Muller (1974) solar atmosphere and found (Fe) = 7.64 ± 0.03 from the Fe I lines; although the Fe II line results indicated (Fe) = 7.53 dex.

When Blackwell et al (1995) computed iron abundances from the Kurucz (1992 unpublished) solar model, Fe I and Fe II lines gave better agreement, at 7.57 and 7.54 dex, respectively, but they claimed that the Kurucz model results are not valid because the solar limb darkening is not reproduced by the model. Blackwell et al (1995) concluded that neither the empirical Holweger-Müller model, nor the Kurucz theoretical model atmosphere, is adequate for measuring the solar iron abundance.

Holweger et al (1995) contested Blackwell et al (1995) claim and argued that Fe I lines analyzed with the Holweger-Müller model give (Fe) =7.48 ± 0.05, or 7.51 with the 0.03-dex non-local thermodynamic equilibrium (non-LTE) correction. Holweger et al (1995) found the same low solar iron abundance from both Fe I and Fe II lines in their analysis.

Lambert et al (1995a) found that the gf values of lines common to results of both Holweger et al (1995), Blackwell et al (1995) had zero average difference, which suggests that gf values are not the source of the abundance difference. They attributed the difference mostly to variations in the measured equivalent widths and damping constants. Another low value of the solar iron abundance was found by Milford et al (1994), who found (Fe) = 7.54 ± 0.05 with the Holweger-Müller solar model and new gf values, from weak Fe I lines that are not sensitive to uncertainties in damping constants or microturbulent velocity.

Kostik et al (1996) attempted to resolve the differences between Blackwell et al (1995), Holweger et al (1995). Kostik et al found that the Blackwell et al's equivalent widths are systematically higher than Holweger et al's values; remeasurement by Kostik et al favored the Holweger et al values. Kostik et al also found suspicious trends in the gf values of the Holweger et al (1995) study, and they agree with Grevesse & Noels (1993) that the spread in iron abundance is dominated by uncertainties in the gf values. They also noted that uncertainties in the microturbulent velocity and collisional damping constants are extremely important to the adopted value. Kostik et al provide a best estimate of the solar iron abundance of 7.62 ± 0.04, which favors the high solar iron abundance; although little weight was placed on the significance of this result.

Anstee et al (1997) measured the solar iron abundance from profile-matching 26 strong Fe I lines, using accurate laboratory collision-damping constants and gf values. They found (Fe) = 7.51 ± 0.01 in complete agreement with the meteoritic iron abundance of Anders & Grevesse (1989), independent of nonthermal motions in the photosphere. Anstee et al traced the discrepancies between previous studies to the use of different atomic data, measured equivalent widths, and assumed microturbulent velocity.

It now seems that the weight of the evidence favors the low value of the solar iron abundance, and the issue may finally be settled; however, this statement has been made before ....